The present invention relates to a fan speed detector used to detect the revolving speed of a cooling fan, and more particularly to such a fan speed detecting circuit which is comprised of a resistor, a diode, a capacitor, and a comparator, and installed in the power circuit of the DC fan motor of the cooling fan to output a pulse frequency wave indicative of the revolving speed of the DC fan motor.
Regular cooling fans for computers, AC adapters or network apparatus, are commonly comprised of a brushless motor, and a fan blade rotated by the brushless motor. As illustrated in FIG. 1, DC power supply is inputted through an oscillator 11 in the cooling fan 1 to a stator 13, causing a rotary magnetic field to be produced at the stator 13 to rotate a rotor 15. Because the cooling effect of the cooling fan 1 directly affects the stability of the electronic product, the revolving speed of the cooling fan must be maintained at a certain range. If the revolving speed is excessively low, the temperature of the electronic components will rise quickly, causing the electronic product unable to function properly. Therefore, it is important to monitor the revolving speed of the cooling fan. Conventionally, photo chopping circuit means is used and installed in the circuit of the motor of a cooling fan to output a signal indicative of the revolving speed of the cooling fan. However, the installation of the photo chopping circuit means complicates the circuit of the cooling fan, and greatly increases the manufacturing cost of the cooling fan.